High-pressure discharge lamps may be high-pressure sodium discharge lamps, and, more specifically, metal halide lamps having improved color rendition. The use of a ceramic discharge vessel for the lamps enables the use of the higher temperatures required for such vessels. The lamps have typical power ratings of between 50 W-250 W. The tubular ends of the discharge vessel are closed by cylindrical ceramic end plugs comprising a metallic current feedthrough passing through the axial hole therein.
Customarily, these current feedthroughs are made of niobium tubes or pins (see U.S. Pat. No. 5,852,952, Juengst et al.). However, they are only partly suitable for lamps that are intended for a long useful life. This is due to the strong corrosion of the niobium material and, possibly, the ceramic material used for sealing the feedthrough into the plug when the lamp has a metal halide fill. An improvement is described in U.S. Pat. No. 4,545,799, to which the European Patent Specification EP-PS 136 505 corresponds. A niobium tube is tightly sealed into the plug by the shrinking process of the "green" ceramic during the final sintering without ceramic sealing material. This is readily possible because both materials have approximately the same thermal expansion coefficient (8.times.10.sup.-6 K.sup.-1).
Although metals such as niobium and tantalum have thermal expansion coefficients that match those of the ceramic, they are known for having poor corrosion resistance against aggressive fills and they have not yet been available for use as a current feedthrough for metal halide lamps.
Metals such as molybdenum, tungsten and rhenium have a high corrosion resistance against aggressive fills, but a low thermal expansion coefficient. Their use as a current feedthrough is, therefore, highly desirable. However, the problem of providing a gas-tight seal while using such feedthroughs has remained unsolved in the past.
It has already been attempted to use a molybdenum tube as a feedthrough, see U.S. Pat. No. 5,404,078, Bunk, et al. In order to avoid the use of ceramic sealing material which can be corroded by aggressive fill materials, the tube is gas-tightly sintered directly into the plug without any sealing material. This has to be done by a special manufacturing method.
Reference to the contents of U.S. Pat. No. 5,404,078, Bunk et al. is expressly made, especially to the manufacturing method and to the composition of the plug material.
The use of a solid molybdenum pin as a feedthrough in connection with a ceramic vessel and plug, made from alumina, has also been discussed in the past. However, the gas-tightness between the plug and the pin is obtained by using a rather corrosion resistant sealing material (glass melt or ceramic melt) or frit which is filled into the gap between the hole of the plug and the feedthrough (see for example U.S. Pat. No. 4,277,715). Preferably, pin diameters below 600 .mu.m are used.
A detailed discussion of this technique is given in the U.S. Pat. No. 4,475,061.
From DE-A 23 07 191, to which Canadian 964,323 corresponds, and U.S. Pat. No. 4,122,042, a metal halide lamp is known which has a ceramic vessel with an electrically conductive plug made from a cermet consisting of alumina and molybdenum metal. A feedthrough of molybdenum is directly sintered into the plug.
The PCT/DE 92/00372, issued in the U.S. as U.S. Pat. No. 5,484,315, Juengst et al., describes a special filling technique for such lamps using a separate filling bore in the plug for evacuating and filling the discharge vessel. The bore is closed off after filling by means of sealing material, i.e. glass melt or ceramic melt, which, however, is in full contact with the constituents or components of the fill and, unfortunately, tends to react with these components of the fill.